Oh man, one time I changed the pan gasket on my 4wd suburban. Knocked the oil pickup tube a bit, causing a leak at the pickup tube o-ring. Talk about a PITA! To get the pan off you have to drop the front transaxle and a bunch of other things... 5 hours down the drain, on my back with oil dripping on me. So I can sympathize
One thing I learned recently about sheet metal by trial and error is to make sure the metal is *clean*! Otherwise you'll have to burn the gunk out to get decent welds, which has a higher chance of blowing through.
I still haven't gotten how to get good penetration though, so I tend to weld both the front and back of my panels when I can.